The present invention relates to an information storage medium having at least a longitudinal bundle of three contiguous servo tracks. The invention further relates to methods for obtaining a tracking signal between a reading head and an information storage medium having at least a bundle of three contiguous servo tracks. The invention also relates to a method for obtaining a tracking code which combines a transversal tracking code with distinct longitudinal position and/or identification information.
It is known in the field of data storage on information storage media to store and use tracking information. As an example, the French patent FR 9215473 discloses an information storage medium carrying a multitude of adjoining longitudinal tracks which contain data. In a data storage and/or retrieval system used in conjunction with this information storage medium it may be necessary to perform transversal tracking to keep a writing and/or reading head aligned with the longitudinal tracks i.e. to detect transversal offset between the writing and/or reading head and the tracks. Transversal tracking information is combined with the data in the tracks by imposing special properties to a data coding. More specifically the data is coded in a way that in a track a digital sum value of the high and low bits making up the data results in a decreasing or increasing direct current (DC) component. Bundles of tracks having a decreasing DC-component alternate transversely with bundles of tracks having an increasing DC-component. The bundles may easily be detected during readout and the writing and/or reading head accordingly positioned with the precision of one trackwidth. In order to achieve an even more precise transversal positioning during readout a further method is used which consists in detecting crosstalk, i.e. monitoring to which extent data contained in two neighboring contiguous tracks contributes to one reading signal. Thus two crosstalk coefficients are computed for each track having two contiguous neighbouring tracks and variations of these crosstalk coefficients indicate transversal offset between the writing and/or reading head and the tracks.
In addition to transversal tracking it may also be necessary to perform longitudinal tracking to determine a longitudinal position on the track. Such longitudinal tracking is not described in FR 9215473.
It is the aim of the presentation to provide a solution for transversal tracking in which the processing of all data tracks as in prior art is not required. Additionally the present invention aims at finding a solution for combining longitudinal position information together with information needed for transversal tracking such that both may be retrieved using a same reading head.